For example, in a hydraulic system for driving a working machine of a wheel loader which is a vehicle for construction work, there is the case where a hydraulic pressure is required, but only a small discharge amount is required at the time of an excavating operation or the like. If a fixed displacement type hydraulic pump is used in such a case, a large amount of pressure oil is returned into a tank, and large power loss is caused. In order to reduce the power loss, there is provided a method of reducing a pump discharge amount at the time of an excavating operation by using a variable displacement type hydraulic pump. As one example of this, there is the one disclosed in the U.S. Pat. No. 6,073,442. According to this, the method is for controlling the pump displacement to reduce to a predetermined displacement of the maximum displacement or less by determining that the working vehicle is under excavating operation when at least one of the following conditions is satisfied: i) the transmission is in the forward and first speed gear position, ii) the working machine is in the excavating position and iii) the vehicle traveling speed is at the set speed or lower.
Among the above described conditions, the excavating position of the working machine is specified as shown in FIG. 13. FIG. 13 is a side view of a working machine 70 in the excavating position. A base end portion of a lift arm 72 is swingably attached to a vehicle body 71 with an arm pin 73, and the vehicle body 71 and the lift arm 72 are connected by a lift cylinder 74. When the lift cylinder 74 is extended or contracted, the lift arm 72 swings around the arm pin 73. A bucket 75 is swingably attached to a tip end portion of the lift arm 72 with a bucket pin 76, and the vehicle body 71 and the bucket 75 are connected via a tilt cylinder 77 and a link device 78. When the tilt cylinder 77 is extended or contracted, the bucket 75 swings around the bucket pin 76. As for the excavating position of the working machine 70, the line Y to Y which connects the arm pin 73 and the bucket pin 76 is set as the reference, and the case where the lift arm 72 is located below the line Y-Y is defined as being in the excavating position.
However, in the above described method, the following problems exist. First, when the transmission is in the forward and first speed gear, the pump capacity is reduced to a predetermined displacement which is the maximum displacement or less. However, in this case, excavating operation is not always performed, but the working vehicle is approaching a predetermined place while operating the working machine with the forward and first speed gear in some cases. In such a case, the speed of the working machine becomes slow, and the working efficiency sometimes reduces. Depending on the soil property, an operation is sometimes performed with the forward and second speed gear, on which occasion, the pump displacement is not reduced and therefore, power loss occurs.
Secondarily, when the vehicle traveling speed is a set speed or lower, the pump displacement is reduced to a predetermined displacement which is the maximum displacement or less. However, there is the case where the working vehicle moves to a destination at the set speed or lower while operating the working machine without performing an excavating operation. In such a case, the pump capacity is also reduced, and the speed of the working machine becomes low, which lowers the working efficiency in some cases. Thirdly, when the transmission is in the forward and first speed gear, the working machine is in the excavating position and the vehicle traveling speed is the set speed or lower, the pump displacement is reduced to the predetermined displacement which is the maximum displacement or less. At the time of ordinary excavation, the bucket is slightly lifted from the ground to prevent the bucket from contacting the ground and increasing the traveling resistance until it comes just in front of the target object, and the bucket is quickly brought into contact with the ground just before it thrusts into the target object. In this case, the response speed of the working machine becomes low, and there arises the problem that the operation slows down and the operator feels incompatibility.